The present invention relates to portable spectrometers, particularly to mechanically cooled portable spectrometers, and more particularly to an actively driven thermal radiation shield for a mechanically cooled portable germanium gamma-ray spectrometer.
Solid-state gamma ray detectors, such as used in x-ray and gamma ray spectrometers, use high purity germanium (GE) crystals that must be cooled to low temperatures (xcx9c100K) for operation. This cooling is usually provided by a cold finger using a liquid cryogen, such as liquid nitrogen (LN). Typically, commercial solid-state detectors use at least 1 liter of the cryogen per day, making long-term unattended use of these detectors impractical. Thermal shielding has been considered to reduce the consumption of the cryogen caused by heat transmitted from the environment to the cooled detector.
Heat is transmitted from the environment to a cooled object in three ways; by convection, conduction, and radiation. For a cooled gamma ray detector, convection is the most significant, and is eliminated by placing the detector in a vacuum chamber. Conduction is minimized by careful construction of the mechanical support for and electrical connections to the detector.
The radiative heating of a detector inside a container is governed by the Stefan-Boltzmann equation:
E=F"sgr"xcex5Cxcex5D(TC4xe2x88x92TD4)
Where E is the heating power in watts, F is a form factor accounting for geometry, "sgr" is the Boltzmann constant, xcex5C and xcex5D are the emissivities of the container and detector, respectively, and TC and TD are the temperatures of the container and detector. The heating power can be dramatically reduced by reducing the temperature of the container, TC. However, practical considerations make the direct reduction of the container temperature impractical in most cases. However, enclosure of the detector in a thermal shield achieves the same effect. Since TD is much less than TC, the power radiated from the thermal shield to the detector is reduced by approximately a factor
ES/E=TS4/TC4
where ES is the power radiated from the thermal shield and TS is the temperature of the thermal shield.
As a typical example, consider a container at room temperature (300K) and a thermal shield at xe2x88x9220xc2x0 C. (253K). The relative radiative load on the detector is (253/300)4=0.51, or a reduction of a factor of 2. Current methods of thermal shielding involve intricate layering of aluminized mylar sheets, typically yielding only a 10-15% reduction in the thermal radiative load. However, this complicates the residual vacuum. Such small reduction is not sufficient to enable long-term unattended use of the detectors. Thus, there has been a need for a way of substantially reducing the amount of heat lift required to maintain mechanically-cooled Ge detectors, and thus enable long-term unattended use.
The present invention provides a solution to the above-referenced problem by providing a mechanically-cooled Ge detector that requires only electrical power to maintain the required operating temperature via an electrically-powered thermal radiation shield. The electrically-powered thermal shield for solid-state gamma-ray detectors dramatically reduces the cooling power requirement for Ge detectors. Thus, since the thermal shield of the present invention can dramatically decrease the power required to cool the detector, it makes low-power mechanically cooled and long-lifetime nitrogen-cooled detectors possible. The thermal shield is constructed of a thin high-thermal-conductivity, low-Z material which maintains the entire shield at a uniform temperature, while minimizing gamma-ray absorption by the shield. The shield and inside wall of the container may be polished or gold-plated, for example, to minimize emissivity.
It is an object of the present invention to provide cooled solid-state gamma-ray detectors.
A further object of the invention is to provide a thermal shield for a cooled solid-state gamma-ray detector.
A further object of the invention is to provide a lower ultimate detector temperature using an electrically-powered thermal shield.
Another object of the invention is to provide a solid-state germanium gamma-ray detector with a thermal shield cooling arrangement achieved by use of a Peltier thermoelectric cooler, or alternatively, a tap midway on the cooler cold finger.
Another object of the invention is to provide a thermal shield for a germanium detector having low power consumption enabling battery power or solar power capability.
Another object of the invention is to provide a cooling shield for a detector which is constructed of thin high-thermal-conductivity, low-Z material to maintain a uniform temperature and minimize gamma-ray absorption.
Another object of the invention is to provide an electrically-powered thermal shield for solid-state gamma-ray detectors that reduces the cooling power requirements which can be used with conventional detectors cooled by liquid cryogen and a cold finger as well as mechanically-cooled detectors, thus making low-power mechanically cooled and long-lifetime liquid cryogen-cooled detectors possible.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawing. The present invention involves a thermal shield for cooled solid-state gamma-ray detectors. The thermal shield is located intermediate the detector and the vacuum enclosure, and is electrically-powered. Cooling of the thermal shield is achieved by use of a Peltier thermoelectric cooler or by a tap located midway on the cooler cold finger, and the power to operate the cooler is typically about 3 watts, which can readily be provided by battery power. The thermal shield is especially advantageous for mechanically-cooled detectors, for which the cooling power to the detector itself is extremely limited, and for which the entire cooling system is electrically powered. As a typical example, with a container at room temperature (300K) and a thermal shield at xe2x88x9220xc2x0 C. (253K), the relative radiative load on the detector is (253/300)4=0.51, or a reduction of a factor of 2. The thermal shield is constructed of a thin (0.060xe2x80x3 to 0.085xe2x80x3) high-thermal-conductivity, low-Z material, such as aluminum, magnesium or beryllium. The high thermal conductivity maintains the entire shield at a uniform temperature, while the low atomic number (low-Z) minimizes gamma-ray absorption by the shield. Additionally, the shield should be as thin as possible to minimize gamma-ray absorption, and polished or gold-plated, for example, to minimize emissivity.